1. Field of the Invention
The present invention relates to a sheet processing device and an image forming system. Specifically, the invention relates to the sheet processing device having a function of performing folding processing on a sheet-like recording medium (hereinafter, in the specification, simply referred to as a “sheet”) such as paper, recording paper, and transfer paper, and the image forming system including the sheet processing device.
2. Description of the Related Art
Conventionally, among post-processing devices that are used in combination with an image forming apparatus such as a copying machine, there is a post-processing device that stitches the sheet center portion(s) of one or a plurality of sheets and folds the center portion of a sheet bundle by a pair of folding rollers installed in parallel in the sheet folding direction so as to bind a saddle-stitched booklet.
Furthermore, also known is an additional folding technique in which an additional folding roller is moved along a fold line after saddle stitching-center folding so as to strengthen the fold line of a saddle-stitched book after the folding processing.
Known is an invention disclosed in Japanese Laid-open Patent Publication No. 2009-143674 as the additional folding technique. This invention relates to a sheet folding device including a conveying unit that conveys a transported sheet or sheet bundle, a first folding unit that performs folding processing on the sheet or the sheet bundle conveyed by the conveying unit, and a second folding unit that reciprocates on the sheet or the sheet bundle subjected to the folding processing by the first folding unit in the direction substantially orthogonal to the sheet conveyance direction so as to perform additional folding on the sheet or the sheet bundle. The sheet folding device further includes a switching unit that switches the conveying unit between a state where a conveyance force can be transmitted to the sheet or the sheet bundle and a state where the conveyance force cannot be transmitted to the sheet or the sheet bundle, and a common driving source that drives the switching unit and the second folding unit. In the sheet folding device, the first folding unit is constituted by a pair of rollers for folding while the sheet or the sheet bundle passes through a roller nip and a plate that presses the sheet or the sheet bundle into the roller nip. Furthermore, the second folding unit is constituted by a plate-like sheet supporting member that supports the sheet or the sheet bundle subjected to the folding processing from the lower side and a pressurized roller that moves on a fold line of the sheet or the sheet bundle located on the sheet supporting member along the fold line.
In the invention as described in Japanese Laid-open Patent Publication No. 2009-143674, the sheet discharged from an image forming apparatus is subjected to center folding processing of folding the sheet in half and stitching, and then saddle stitching binding processing of performing half-folding processing. Subsequently, the additional folding roller (second folding unit) is moved along the sheet fold portion to pressurize the sheet fold portion against the supporting plate supporting the lower surface side of the sheet. In this manner, the fold portion of the saddle-stitched book after the folding processing is strengthened.
In the configuration in which the sheet bundle supported on the supporting plate is folded by the additional folding roller, a pair of additional folding rollers cannot be moved along the fold portion of the sheet bundle because of the supporting plate. For this reason, additional folding by using the pair of additional folding rollers cannot be executed.
When the supporting plate is not provided, there arises a possibility that drooping of a sheet front end and drooping of a sheet rear end are occurred. The occurrence of the drooping of the sheet front end and the drooping of the sheet rear end causes a problem in conveyance performance of the sheet bundle. When the conveyance performance of the sheet bundle is bad, an additional folding position is deviated or the sheet bundle deflects at the time of the additional folding, resulting in lowering of folding quality of the fold portion in some cases.
In view of the above-mentioned conventional problems, there is need to perform additional folding on a fold portion reliably while guaranteeing folding quality of the fold portion.